At the RSA conference, researcher Dan Kaminsky will show how a Web-based attack could be used to get behind the firewall and seize control of widely-used routers

On Tuesday, researcher Dan Kaminsky will show how a Web-based attack could be used to seize control of certain routers.

Kaminsky has spent the past year studying how design flaws in the way that browsers work with the Internet's DNS (Domain Name System) can be abused in order to get attackers behind the firewall. But at the RSA Conference in San Francisco, he will demonstrate how this attack would work on widely used routers, including those made by Cisco's Linksys division and D-Link.

The technique, called a DNS rebinding attack, would work on virtually any device, including printers, that uses a default password and a Web-based administration interface, said Kaminsky, who is director of penetration testing with IOActive.

Here's how it would work. The victim would visit a malicious Web page that would use JavaScript code to trick the browser into making changes on the Web-based router configuration page. The JavaScript could tell the router to let the bad guys remotely administer the device, or it could force the router to download new firmware, again putting the router under the hacker's control.

Either way, the attacker would be able to control his victim's Internet communications.

The technical details of a DNS rebinding attack are complex, but essentially the attacker is taking advantage of the way the browser uses the DNS system to decide what parts of the network it can reach.

Although security researchers had known that this type of hack was theoretically possible, Kaminsky's demo will show that it can work in the real world, said David Ulevitch, CEO of DNS service provider OpenDNS. "I'm always a fan of when something that's theoretical gets made real, because it makes people act," he said.

On Tuesday, OpenDNS will offer users of its free service a way to prevent this type of attack, and the company will also set up a Web site that will use Kaminsky's techniques to give users a way to change the passwords of vulnerable routers.

The attack "underscores the need for people to be able to have more intelligence on the DNS," Ulevitch said.

Although this particular attack takes advantage of the fact that routers often use default passwords that can be easily guessed by the hacker, there is no bug in the routers themselves, Kaminsky said. Rather, the issue is a "core browser bug," he said.

Router makers have known for some time how their default passwords can be misused by attackers. Three months ago, hackers showed how a similar attack could be launched, exploiting a flaw in the way Universal Plug-and-Play works on PCs.

Cisco tries hard to discourage Linksys customers from using routers with default passwords, said Trevor Bratton, a company spokesman. "One of the first things that our setup software does is change that default name," he said. "So anyone who does as we ask with the initial setup will be prompted to change that."

The problem is that home users rarely follow this advice, Kaminsky said. "The vast majority of home users have a device with a default password," he said.